Rig a zbrush model in blender and send it back to zbrush with GOB?
Hmm. Try importing the obj you exported from blender back into blender. My guess is that it will be in the same pose that you exported from Zbrush, but all triangles because your pose was not applied to the mesh before export.

I haven't used GOB in a while, but it should work so long as you didn't change the topology and iirc it should work even if you did change the topology. You need to actually apply the armature pose to your model in blender so that the armature modifier no longer exists. There should be an OBJ option to apply all modifiers on export.

Also, I would just recommend using in Zbrush, and the masking tool. That would be way less complex, and if you didn't know how to do those things, increase your skills in Zbrush. It's also better because you can use layers for per-bodyparty-poses or entire poses. Right?

Edit: Yeah, I haven't done this in years, but I still remember. Just change your high poly Ztool to low poly, press the GoZ button to export to blender (blender needs to have the import button active), modify the mesh, export the mesh as a Wavefront OBJ from Blender. Import the obj to Zbrush, it will apply your changes instantly.